1. Field of the Invention
The invention relates to reinforced concrete, and more particularly to a steel fiber reinforced concrete with increased flexural strength, and to steel fibers for use therein.
2. Discussion of the Related Art
Steel fibers for the reinforcement of concrete are generally known. Such fibers are introduced into a mixture for the preparation of concrete and well mixed until they are equally distributed in the mixture. After hardening of the concrete, they act as a reinforcement that strengthens the concrete.
With a view to being adapted to mixing and for reinforcement, steel fibers have in general a thickness in the range of 0.3 to 1.2 millimeters, most usually in the range from 0.5 to 1 millimeter. They have generally a length-to-thickness ratio in the range from 30 to 150, most usually in the range from 50 to 100. They have generally a tensile strength in the range from 500 to 1600 Newtons per square millimeter, most usually from 900 to 1300 N/mm.sup.2. When less than 500 N/mm.sup.2, then the fibers exhibit too low a resistance to deformation at rupture of the concrete, such that the concrete would then exhibit a brittle behaviour at rupture. Conversely, when more than 1300 N/mm.sup.2, the brittle behaviour of the concrete is sufficiently avoided, but the increased tensile strength of the steel does not further increase the flexural strength of the concrete.
The reinforcing effect of the steel fibers manifests itself specifically in the increase of the flexural strength of the concrete. The flexural strength is the tensile strength of the concrete at rupture of the beam, in a concrete beam under flexural loading and at the location where the maximal tension appears. The so-called modulus of rupture is representative of the flexural strength. The modulus of the rupture is the value .delta. obtained by the formula: EQU .delta.=P.times.L.times.B/H.sup.2,
in which:
L=the span length between the two supporting points of a test beam that is loaded in flexure by means of a load that for one half-load acts at a distance of one-third of the span length from one supporting point, and for the other half-load acts at a distance of one-third of the span length from the other supporting point; PA1 B=the breadth of that test beam; PA1 H=the height of that test beam; and PA1 P=the sum of the above-mentioned half-loads, at rupture. PA1 B=a constant that depends on the degree of anchoring and on the orientation of the fibers in the concrete, and that, in first approximation, is independent of the tensile strength of the steel of the fiber; PA1 p=the percentage by volume of fibers in the concrete; and PA1 L/D=the length-to-diameter ratio of the fiber used.
The value obtained by this formula corresponds in fact to the tension at rupture in the part of the beam in the region under tension which is the most distant from the neutral plane, calculated as if that rupture would still be located in the linear part of the stress/strain curve. Due to the presence of the fibers, however, the concrete still does not show a brittle rupture after the first crack, but the stress/strain curve raises further in a non-linear manner towards a maximum, with the result that the concrete shows a post-crack resistance that is considerably higher than the first-crack resistance. In this way, the fibers produce a considerably higher increase of the flexural strength, as observed via the modulus of rupture.
It is known that the flexural strength produced by the metal fibers is given, in first approximation, by the formula: EQU F=B.times.p.times.L/D (1)
in which:
It is known to use fibers with grappling form, that is, fibers with a shape that differs from the straight shape with constant cross-section over the length, in order to obtain a degree of anchoring of the fibers that is as high as possible. There are, for instance, fibers that are provided with incurvations or undulations, either over the whole or part of their length, or only at the extremities, such as hook-shaped incurvations. Similarly, there are fibers of which the cross-section profile changes over the length, such as thickenings, alternating with thinner parts, or a flattened profile that alternates with a round profile, either over the whole length, or only at the extremities, such as thickenings in the form of a nail head at each of the extremities. These deformations can be used alone or in combination with each other. The increase of the degree of anchoring can be obtained by the use of such fibers with grappling form, and can further be obtained or increased by the roughening of the fiber surface.
Besides the improvement of the degree of anchoring, it is also known that an L/D-ratio should be chosen that is as high as possible, at any rate more than 50. However, when the fibers are made by a cold work-hardening cross-section reduction operation, such as by cold rolling or drawing or elongating, and a ratio is taken above about 120 to 130, then, using an acceptable length of 2.5 to 10 centimeters for the mixing, the diameter unfortunately becomes too small for still being economically acceptable. The manufacturing cost of the fiber per kilogram increases as the fiber is thinner. One can not indefinitely raise the L/D-ratio, as there is a such a limitation.
When willing to further drive up the flexural strength of the concrete, using such an optimalized fiber for efficiency, one should then be willing, according to formula (1), to introduce in the concrete a portion p of fibers that is as high as possible. Again there is also a practical limitation, determined by the mixability of the fibers. The higher indeed the L/D-ratio, the more difficult to mix the fibers together in the concrete without danger of balling-up. This means that a higher L/D-ratio corresponds to a lower maximum percentage in volume that can be mixed in the concrete. The limit of mixability can be determined by experiment, as for example shown in U.S. Pat. No. 4,224,377, by the approximative formula : EQU p.times.(L/D).sup.1.5 =maximum 1100,
wherein this maximum value can be increased to a certain extent by special measures taken to improve the mixability, such as introducing the fibers in a form where they are glued together, as known from the above U.S. patent.